In general, a molded product such as a tablet or food has been conventionally required to be substantially uniform in weight or thickness. It is difficult to measure the amount of powder material before being molding per molded product in a powder material compression molding machine for fabricating numerous molded products in a short period of time, like a continuously tablet compressing machine. In view of this, a pressure during compression has been conventionally detected, and then, the amount of powder material to be inserted into a die has been automatically controlled such that the detected pressure becomes a certain reference pressure previously calculated in accordance with a target amount of powder material in a powder material compression molding machine for inserting a punch into a die deeply to a predetermined position so as to compressively mold a powder material in consideration of an approximately one-to-one relationship between a weight of a molded product and a pressure in compressing a powder material.
However, there may occur a fear of variations of the weight of a molded product since the above-described one-to-one relationship is broken up due to expansion or shrinkage caused by heat generated at the die or the punch, fluctuations in distribution of a grain size of a powder material or fluctuations in flow even if a tablet compressing pressure is controlled to become the reference pressure in a method for indirectly controlling the weight of the molded product in the above-described manner. Therefore, there has been conventionally known a controlling method disclosed in, for example, Japanese Patent No. 2975346 (corresponding U.S. Pat. No. 6,325,609), in which a tablet compressing pressure and the weight and thickness of a molded product are measured; the amount of powder material is adjusted based on the tablet compressing pressure; the amount of powder material is adjusted based on the measured weight of the molded product during the stoppage of the adjustment of the amount of powder material based on the tablet compressing pressure; and the tablet compressing pressure is adjusted based on the measured thickness of the molded product. The controlling method disclosed in Japanese Patent No. 2975346 is configured such that when an actually measured weight and an actually measured thickness fall within an allowable range, a previously calculated reference pressure is calibrated in accordance with a predetermined reference amount of powder material based on an actually measured pressure.
In the above-described powder material compression molding machine, the weight and thickness of the product are related to each other such that the control, that is, the adjustment of either one of the weight and thickness at the time of fabrication exerts an influence on the other. In the controlling method disclosed in Japanese Patent No. 2975346, the weight and thickness of the product are measured at the same time when the product is sampled, and then, both of the weight and the thickness are controlled based on the measurement result, so that the reference pressure is calibrated. However, as described above, if the length of the punch, for example, is increased due to an increase in heat inside of the powder material compression molding machine after the start of the fabrication, the tablet compressing pressure is increased while the thickness is decreased. Here, the calibration of the reference pressure possibly induces an inconvenience. In other words, in the case where the punch expands with the same weight of the product, the tablet compressing pressure is increased while the thickness is decreased. When the thickness is decreased, a compression position is adjusted in such a manner that the thickness approximates to a reference thickness, thereby varying the tablet compressing pressure. In such a case, if the reference pressure is calibrated based on the tablet compressing pressure at that time, the weight is unfavorably changed in order to adjust the amount of powder material based on the calibrated reference pressure.